I. FIELD OF THE INVENTION
This invention relates to a process and an apparatus for reclaiming and/or concentrating waste aqueous solutions of gas treating chemicals.
II. DISCUSSION OF THE PRIOR ART
Natural gas contains a number of acidic gaseous components, such as hydrogen sulfide and carbon dioxide, as well as water vapor, which should desirably be removed from the gas before it is transported and/or used.
These components can be removed from natural gas, and other similar gases if desired, by contacting the gas in countercurrent flow with an aqueous solution of a gas treating chemical, usually an alkanolamine such as monoethanolamine (MEA), diethanolamine (DEA) or methyl diethanolamine, or a glycol such as mono-, di- or tri-ethylene glycol, or sulfinol. The solution of the gas treating chemical efficiently absorbs the acid components or water from the natural gas. Thereafter, the solution of the gas treating chemical is regenerated by stripping the absorbed acid materials from the solution so that the solution can be recirculated and re-used for the treatment of further natural gas. This stripping operation is normally brought about by flowing the solution countercurrent through steam in a regenerator or stripper apparatus.
Over a period of time, certain contaminants build up in the gas treating solution subjected to repeated use in this way to the extent that the efficiency of removal of the acidic gaseous components is reduced or the gas treating solution becomes too viscous to pump efficiently. When this occurs, the solution is generally replaced by a fresh aqueous gas treating chemical solution. However, this gives rise to two disadvantages. Firstly, the resulting large quantities of waste solution are difficult and expensive to dispose of safely because of their content of toxic substances. Secondly, the cost of fresh gas treating chemical is quite high, so the overall cost efficiency of the process is reduced. In view of this, attempts have been made to reclaim contaminated (waste) aqueous solutions of gas treating chemicals so that they can continue to be used.
The contaminants commonly found in waste solutions of gas treating chemicals include products of the thermal degradation of the gas treating chemical, heat-stable salts, asphaltenes, light hydrocarbons, suspended solids or combinations thereof. The degradation products are high boiling nitrogen and oxygen compounds, and the heat-stable salts include sodium thiosulfate, sodium thiocyanate and sodium sulfide. In order to reclaim the waste solutions of gas treating chemicals, it would therefore be necessary to remove all such impurities from the solutions while leaving the gas treating chemical substantially intact. One way to achieve this would be to subject the waste solution to evaporation in order to liberate water vapor and vaporized treating chemical, leaving the contaminants in an unvaporized liquid residue. However, this is made difficult by the fact that all of the gas treating chemicals used for the treatment of natural gas are susceptible to decomposition at elevated temperatures. In the case of the alkanolamines, which are the preferred gas treating chemicals, any attempt to separate contaminants from the solutions by heating the solutions to temperatures above about 400.degree. F. will result in significant thermal decomposition of the gas treating chemical itself. While the vaporization temperature of the waste solution can be reduced by carrying out the evaporation under a vacuum, it is still difficult to avoid heating parts of the solution to temperatures above 400.degree. F. if rapid recovery of chemical from the waste solution is to be achieved.
Moreover, another problem that is encountered is that the gas treating chemicals, and particularly the alkanolamines, are corrosive and their corrosive activity increases with temperature. The corrosion problem can be serious in those parts of the equipment where temperatures are high and residence times are long.
This corrosion problem has given rise to the conventional wisdom that, when dealing with aqueous solutions of alkanolamines and other gas treating chemicals (a) the temperature should be limited to the range of 260.degree. F. to 300.degree. F., (b) the concentration of the alkanolamine should be limited to 30 weight percent, and (c) that stainless steel should be used for critical parts of the apparatus, i.e. those parts of the apparatus where the solution is heated and the residence times are long. For example, in "Gas Purification" by Kohl and Reisenfeld, 4th Edition, 1985, Gulf Publishing, page 121, it is stated in connection with ethanolamine gas purification that: "The authors concluded from the results of this study that all vessels and exchanger shells in the treating plant can be constructed of carbon steel. However, stainless steel, type 304, is recommended for the hottest amine exchanger pass, the boiler, the amine cooler, the amine reclaimer, and certain sections of the piping."
However, it will be appreciated that the use of stainless steel for the gas treating chemical reclaimer necessarily significantly increases the capital cost of the apparatus compared to similar apparatus made of carbon steel.
It is therefore difficult to design apparatus for reclaiming waste aqueous solutions of gas treating chemicals that can be operated both efficiently and at reasonable cost for prolonged periods of time.
A further problem is that environmental considerations are very important nowadays and gas treating chemicals, as well as their contaminants and decomposition products, are generally regarded as pollutants. It is therefore essential to ensure that any effluent from the apparatus that is intended to be discarded by dumping meets stringent anti-pollution standards.
It was stated above that there is an alternative to reclaiming waste aqueous gas treating chemical solutions, i.e. merely to discard them and to use fresh solutions, but that this gives rise to disposal problems. One way of reducing such problems would be to concentrate the waste solution by evaporating off excess water and thus reducing the quantity of hazardous material to be disposed of. Since, in such cases, there would be no intention of re-using the gas treating chemical, it might be thought that there would be no disadvantage in subjecting the chemical to temperatures above its decomposition point. This, however, is not the case because the resulting decomposition products make the solution viscous, and hence more difficult to pump, and because the higher temperatures and concentration levels of the gas treating chemical and decomposition products increase the risk of serious corrosion of the apparatus.